1. Field of Invention
This invention relates to waveguides. Specifically, the present invention relates to baluns for canceling common mode electromagnetic energy in differential input signals or for providing differential output signals lacking common mode energy in response to an input signal.
2. Description of the Related Art
A balun converts unbalanced transmission line inputs into one or more balanced transmission line outputs or visa versa. Baluns are employed in various demanding applications including output stages of delta sigma modulator Direct Digital Synthesizers (ΔΣ DDS) and antenna feeds. Such applications demand miniature, wide-bandwidth (wideband) baluns compatible with integrated circuits and capable of rejecting common mode energy from differential inputs or providing differential outputs lacking common mode energy.
Space-efficient, wideband baluns are particularly important in ΔΣ DDS applications, where dual wideband differential lines must often be converted to a single line output. ΔΣ DDS's are often employed to generate analog output signals with desired amplitudes, frequencies, and phases based on certain digital inputs. ΔΣ DDS's are employed in various applications, including active pulse radar and digital wireless communications, to facilitate signal waveform generation for signal mixing, up-converting, down-converting, frequency synthesis, and signal offsets.
A conventional ΔΣ DDS employs a 1-bit Digital-to-Analog Converter (DAC) to selectively sample an analog input signal to produce a corresponding digital output signal. The DAC must have a relatively high sampling rate to compensate for the low 1-bit resolution quantizer. Consequently, the output of the 1-bit DAC is often a high-frequency pulse-like signal. This 1-bit DAC output is typically filtered to remove quantization noise.
1-bit DACs employed in ΔΣ DDS's often provide dual pulse-like output signals, which are 180 degrees out of phase. These differential pulse-like signals may occur over a wide frequency range and must be converted to a single output via a balun. The 1-bit DAC includes transistors, which often have slightly different rise and fall times. Differences in transistor rise and fall times create undesirable common mode components in pulsed output signals. For optimum DDS performance, these common mode components must be rejected in the final ΔΣ DDS output.
Conventionally, wire-wound ferrite baluns are employed to convert differential input lines into a single balanced output transmission line. These baluns have iron cores wrapped in wire and act as power transformers. Unfortunately, ferrite baluns are bandlimited at lower frequencies, typically cutting off frequencies beyond two or three gigahertz, which is undesirably low for many ΔΣ DDS applications. Furthermore, ferrite baluns are more suitable for continuous wave applications and less suitable for pulse applications, as ferrite baluns are often susceptible to reflections resulting from fast input pulses. To improve balun transient response, the baluns are made larger. The large ferrite baluns are difficult to incorporate into miniature ΔΣ DDS integrated circuits and poorly reject common mode energy.
Alternatively, baluns are constructed using various waveguides having quarter wavelength sections. Unfortunately, use of quarter wavelength sections may result in undesirably large baluns. In addition, these baluns are relatively narrow-banded and are susceptible to large reflections when fed with pulsed inputs.
Hence, a need exists in the art for a miniature wideband balun that is easily incorporated in integrated circuits and that efficiently rejects common mode energy from differential pulsed inputs and provides a balanced output. Such a balun can also provide balanced differential outputs lacking common mode energy from a balanced input. There exists a further need for an efficient ΔΣ DDS that incorporates the efficient wideband balun.